Method of preparing positive active material for a lithium secondary battery

ABSTRACT

A process of manufacturing a positive active material for a lithium secondary battery includes adding a metal source to a doping element-containing coating liquid to surface-treat the metal source, wherein the metal source is selected from the group consisting of cobalt, manganese, nickel, and combination thereof; drying the surface-treated metal source material to prepare a positive active material precursor; mixing the positive active material precursor with a lithium source; and subjecting the mixture to heat-treatment. Alternatively, the above drying step during preparation of the positive active material precursor is substituted by preheat-treatment or drying followed by preheat-treatment.

CLAIM OF PRIORITY

[0001] This application makes reference to, incorporates the sameherein, and claims all benefits accruing under 35 U.S.C. §119 from anapplication for METHOD OF PREPARING POSITIVE ACTIVE MATERIAL FOR ALITHIUM SECONDARY BATTERY earlier filed in the Korean IntellectualProperty Office on 13 May 2002 and there duly assigned Serial No.2002-26200.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a process of preparing apositive active material for a lithium secondary battery, and morespecifically, to a process of preparing a positive active material for alithium secondary battery with high discharge potential, high powerdensity, high rate capability, and good cycle life characteristics.

[0004] 2. Description of the Related Art

[0005] Generally, rechargeable lithium batteries use a material from orinto which lithium ions are deintercalated or intercalated for positiveand negative active materials. For the electrolyte, an organic solutionof a lithium salt or a lithium ion-conducting polymer is used. Arechargeable lithium battery produces electrical energy as a result ofchanges in the chemical potentials of the active materials during theintercalation and deintercalation reactions of the lithium ions.

[0006] Among the active materials which have been considered for theactive material of negative electrodes of batteries, lithium metal givesboth high cell capacity and high voltage because it has a highelectrical capacity per unit mass and relatively high electronegativity.However, since it is difficult to assure the safety of a battery usinglithium metal, a carbonaceous material that is capable of intercalatingand deintercalating lithium ions is used extensively for the activematerial of the negative electrodes in lithium secondary batteries.

[0007] Lithium metal compounds of a complex formula are often used as apositive active material of the lithium secondary battery. Typicalexamples include lithium-containing compounds such as LiCoO₂, LiMn₂O₄,LiNiO₂, LiNi_(1−x)Co_(x)O₂(0<x<1), and LiMnO₂. Manganese-based positiveactive materials such as LiMn₂O₄ or LiMnO₂ have relatively good safetyproperties, are less costly than the other materials, and areenvironmentally friendly. However, these manganese-based materials havea disadvantage of a relatively low capacity. LiNiO₂ has the highestdischarge capacity of all the positive active materials mentioned above,but it is difficult to synthesize and it is the least thermally stableamong the compounds mentioned above. LiCoO₂ has many technicaladvantages over the other materials, such as relatively good cycle lifeand relatively high specific energy. Accordingly, this compound ispresently the most popular material for positive electrodes ofcommercially available Li-ion batteries, even though its cost isrelatively high.

[0008] These lithium-containing compounds are currently synthesizedusing a solid-phase process. For example, a lithium compound (lithiumsource) such as LiOH or Li₂CO₃ and a cobalt compound (cobalt source) aremixed at a desirable equivalent ratio followed by calcinating themixture at a temperature of 800-1000° C. to prepare LiCoO₂. A transitionmetal source may be added to the mixture of the lithium source andcobalt source prior to the calcination to improve charge/dischargecharacteristics of the LiCoO₂.

[0009] Recently, with an increased demand for portable electronicequipment that is more compact and lightweight, there has been anincreased demand for various types of batteries including a Li-ionbattery with an improved active material that can assure good batteryperformance such as high discharge potential, high power density, highrate capability, and good cycle life characteristics.

SUMMARY OF THE INVENTION

[0010] It is therefore an object of the present invention to provide animproved process of preparing a positive active material for a lithiumsecondary battery with good electrochemical characteristics includingcycle life, discharge potential, and power capability.

[0011] Additional objects and advantages of the invention will be setforth in part in the description which follows, and in part will beobvious from the description, or may be learned by practice of theinvention.

[0012] In order to accomplish these and other objects, one embodiment ofthe present invention provides a process of preparing a positive activematerial for a lithium secondary battery comprising adding a metalsource to a doping element-containing coating liquid to surface-treatthe metal source, wherein the metal source is selected from the groupconsisting of cobalt, manganese, nickel, and a combination thereof;drying the surface-treated metal source material to prepare a positiveactive material precursor; mixing the positive active material precursorand a lithium source; and subjecting the mixture to heat-treatment.

[0013] According to another embodiment of the present invention, thedrying process during a preparation of the positive active materialprecursor is substituted by preheat-treatment or drying followed bypreheat-treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] A more complete appreciation of the invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings, wherein:

[0015]FIG. 1 illustrates a cross-sectional view of a lithium secondarybattery cell;

[0016]FIG. 2 is a graph showing charge and discharge characteristics ata low rate of the battery cells according to Comparative Example 1,Reference Example 3, and Examples 2, 3, 5, and 7 of the presentinvention, respectively;

[0017]FIG. 3 is a graph showing charge and discharge characteristics ata high rate of the battery cells according to Comparative Example 1,Reference Example 3, and Examples 2, 3, 5, and 7 of the presentinvention, respectively;

[0018]FIG. 4A is a graph showing a variance of average charge anddischarge potential of the battery cells according to ComparativeExample 1, Reference Example 3, and Examples 2, 3, 5, and 7,respectively;

[0019]FIG. 4B is an enlarged view of a portion of FIG. 3A; and

[0020]FIG. 5 is a graph showing cycle life characteristics of batterycells according to Examples 2, 5, and 7 of the present invention,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Reference will now be made in detail to the present embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and specific Examples, wherein like referencenumerals refer to like elements throughout. The embodiments aredescribed below in order to explain the present invention by referringto the figures and the specific Examples.

[0022] In the present invention, doping elements are introduced to apositive active material precursor through coating, and the precursorand a lithium-containing compound are mixed followed by heat-treatmentto prepare a positive active material including doping elements.

[0023] Conventionally, a positive active material including dopingelements is prepared through the following processes: a lithium source,a metal source, and a doping element source are mixed in a solid powderstate and heat-treated. However, this method has a shortcoming in thatthe doping elements are not introduced in the positive active materialuniformly and thereby there is a limit for improving electrochemicalproperties of the positive active material produced through this method.The inventor of the present invention has filed Korean Application No.2001-31530, the disclosure of which is incorporated herein by reference,wherein doping elements are introduced to the positive active materialuniformly by coating the positive active material with a dopingelement-containing coating liquid to improve electrochemical performancesuch as high rate properties, power capability, cycle lifecharacteristics, and initial discharge capacity of the positive activematerial. In the above application, the positive active material isproduced through coating of a resulting material with a dopingelement-containing coating liquid. But in the present invention, thepositive active material is produced through coating of an intermediatematerial with a doping element-containing coating liquid.

[0024] Electrodes of a lithium secondary battery are fabricated bycoating a slurry including an active material on a current collector,dried and compressed. In the case that the positive active material is alithium-nickel-manganese-based compound, gelation occurs duringpreparation of the slurry. It is desirable for the slurry for preparingan electrode to have a viscosity of 4000 to 7000 centipoise (cps). Whenthe viscosity is below the lower limit, fluidity of the slurry is largeand desirable coating properties may not be obtained, and when theviscosity is higher than the upper limit, uniform coating cannot beobtained. The positive active material prepared through the presentinvention does not cause gelation during preparation of the slurry, andis advantageous for controlling the viscosity of the slurry to besuitable for coating on the current collector.

[0025] Hereinafter, the process of preparing the positive activematerial of the present invention is explained in further detail. Ametal source (intermediate material) is added to a dopingelement-containing coating liquid to surface-treat the metal source.

[0026] For the doping element, any element can be used if it is solubleor suspendable in an organic solvent or water. According to one example,the doping element is at least one selected from the group consisting ofMg, Al, Co, Ni, K, Na, Ca, Si, Fe, Cu, Zn, Ti, Sn, V, Ge, Ga, B, P, Se,Bi, As, Zr, Mn, Cr, Sr, Sc, Y, a rare earth element (for example La,Ce), or a combination thereof, and it preferably includes Mg, Al, Co,Ni, Fe, Zr, Mn, Cr, Sr, V, or a combination thereof.

[0027] The doping element-containing coating liquid is prepared byadding a doping-element source to a volatile organic solvent or water.In this embodiment of the invention, “coating liquid” refers to ahomogeneous suspension or a solution. The doping-element source includesany doping-element or any doping-element-containing compound that issoluble or suspendable in the organic solvent or water. Thedoping-element source may include a doping-element; adoping-element-containing alkoxide such as methoxide, ethoxide, orisopropoxide; a doping-element-containing salt; or adoping-element-containing oxide.

[0028] A suitable type of doping-element-containing compound can easilybe chosen by one having ordinary skill in the art. For an example of thedoping-element source, tetraethyl orthosilicate may be used as a siliconsource, whereas B₂O₃, H₃BO₃, or HB(OH)₂ can be used as a boron source.HB(OH)₂ is prepared by dissolving B₂O₃ in an organic solvent or waterfollowed by drying the liquid. When vanadium is used as adoping-element, vanadium oxide (V₂O₅) or vanadate such as ammoniumvanadate (NH₄(VO)₃) may be examples of the doping element source.

[0029] Examples of the volatile organic solvents according toembodiments of the invention include, but are not limited to, C₁ to C₄straight or branched alcohols such as methanol, ethanol, or isopropanol.Other solvents according to embodiments of the invention include ether,methylene carbonate, and acetone.

[0030] The amount of the doping element is preferably 0.01 to 20 wt % ofthe coating liquid, preferably 0.01 to 10 wt %. When the amount is below0.01 wt %, the subsequent drying process takes too long, and when theamount of the doping element is more than 20 wt %, it is difficult toprepare a coating liquid suitable for doping.

[0031] The metal source includes a material including cobalt, manganese,nickel, or a combination thereof. Examples of manganese sources includemanganese acetate, manganese dioxide, or manganese nitrate; examples ofcobalt sources are cobalt hydroxide, cobalt oxide, cobalt nitrate, andcobalt carbonate; and examples of nickel sources are nickel hydroxide,nickel nitrate, and nickel acetate, but they are not limited thereto.Fluoride sources, sulfur sources, or phosphorous sources may be furtherintroduced to the metal source together with the manganese sources,cobalt sources, or nickel sources. The fluoride sources may be manganesefluoride or lithium fluoride, and the sulfur sources may be manganesesulfide or lithium sulfide. A phosphorous source may be H₃PO₄. Thesecompounds are preferable examples, but the present invention is notlimited thereto.

[0032] The metal source may include at least two metals. These metalsources can be prepared using a solid phase synthesis method or aco-precipitation method. In the former method, at least two metalsources in a solid powder state are mixed and heat-treated to prepare ametal source including at least two metals. In the latter method, atleast two metal sources in a solution state are mixed and the pH of themixture is controlled to prepare a metal source including at least twometals.

[0033] The metal source as described above is surface-treated (coated)with a doping-element-containing liquid. The coating process may beperformed by a wet-coating method such as a dipping method or a spraymethod. The dipping method includes dipping the metal source in thecoating liquid, removing excess liquid if necessary, and then drying thecoated active material. Generally, the dip coating method is used.

[0034] Subsequent to the wet coating, the coated metal source issubjected to one process selected from drying, preheating, anddrying-and-preheating to prepare a positive active material precursor.

[0035] The drying operation is performed at a temperature in the rangefrom room temperature (i.e., roughly 20° C.) to 300° C., for 1 to 24hours. When the drying temperature is lower than room temperature, thedrying time is unduly prolonged. If the drying temperature is too high,e.g., much higher than 300° C., this process is defined as a preheatingprocess. When the drying duration is too short, e.g., much shorter than1 hour, solvent drying is not sufficient. If the drying duration is toolong, e.g., much longer than 24 hours, the drying process is undulyprolonged.

[0036] The preheating operations are performed at a temperature in therange of 300° C. to 1000° C., for 1 to 24 hours. When the preheatingtemperature is lower than 300° C., this process is defined as a dryingprocess. If the preheating temperature is too high, e.g., much higherthan 1000° C., the volatile doping elements may be removed to change theequivalent composition of the resulting positive active material. Whenthe preheating duration is too short, e.g., much shorter than 1 hour,solvent drying is not sufficient. If the preheating duration is toolong, e.g., much longer than 24 hours, the preheating process is undulyprolonged.

[0037] The “drying-and-preheating” means that the drying operation isperformed under the same condition of the above drying operation, andsubsequently the preheating operation is performed under the samecondition of the above preheating operation.

[0038] Through drying, preheating, or drying-and-preheating process, apositive active material precursor (metal source) surface-treated with adoping element is produced. That is to say, a surface-treatment layercomprising at least one doping-element-containing compound is formed onthe surface of the positive active material precursor. Thedoping-element-containing compound has an amorphous, a semi-crystalline,or a crystalline phase, or a mixture of these phases. Particularly,through preheating or drying-and-preheating process, a solid solutioncompound between the doping element and the metal source may beproduced.

[0039] The surface-treated positive active material and a lithium sourceare mixed at an desirable equivalent ratio and heat-treated. Theheat-treatment process is preferably performed twice. A firstheat-treatment step is preferably performed at about 400 to 500° C. for5 to 20 hours, and a second heat-treatment step is preferably performedat about 700 to 900° C. for 10 to 30 hours. If the first heat-treatmentstep temperature is less than 400° C., the metal sources may not reactcompletely with the lithium sources, and if the first heat-treatmentstep temperature is more than 500° C., the lithium element, which has amelting point of 453.2° C., may be lost. If the second heat-treatmentstep temperature is less than 700° C., the resulting crystallinematerial is not produced, and if the second heat-treatment steptemperature is more than 900° C., the lithium element is evaporatedresulting in a positive active material with an undesirable equivalentratio and excessive crystallinity. The excessive crystallinity mayprevent movement of lithium ions during charge and discharge.

[0040] The first heat-treated positive active material may be cooled toroom temperature and then ground further before the secondheat-treatment to obtain a positive active material, a lithiatedintercalation compound with uniform composition.

[0041] The lithiated intercalation compound produced in accordance withthe above processes may or may not be sieved to obtain a powder with adesirable average diameter.

[0042] Examples of the lithiated intercalation compound are representedin the following formulas (1) to (11):

Li_(x)Mn_(1−y)M′_(z)A₂  (1)

Li_(x)Mn_(1−y)M′_(z)O_(2−a)X_(a)  (2)

Li_(x)Mn_(2−y)M′_(z)A₄  (3)

Li_(x)Co_(1−y)M′_(z)A₂  (4)

Li_(x)Co_(1−y)M′_(z)O_(2−a)X_(a)  (5)

Li_(x)Ni_(1−y)M′_(z)A₂  (6)

Li_(x)Ni_(1−y)M′_(z)O_(2−a)X_(a)  (7)

Li_(x)Ni_(1−y)Co_(z)M′_(w)A_(b)  (8)

Li_(x)Ni_(1−y)Co_(z)M′_(w)O_(2−b)X_(b)  (9)

Li_(x)Ni_(1−y)Mn_(z)M′_(w)A_(b)  (10)

Li_(x)Ni_(1−y−z)Mn_(z)M′_(w)O_(2−b)X_(b)  (11)

[0043] wherein

[0044] 0.9≦x≦1.1; 0≦y≦0.5; 0≦z≦0.5; ≦0≦w≦2; 0≦a≦0.5; 0≦b≦2;

[0045] M′ is at least one doping element selected from the groupconsisting of Mg, Al, Co, Ni, K, Na, Ca, Si, Fe, Cu, Zn, Ti, Sn, V, Ge,Ga, B, P, Se, Bi, As, Zr, Mn, Cr, Sr, and rare earth elements;

[0046] A is at least one element selected from the group consisting ofO, F, S, and P; and

[0047] X is at least one element selected from the group consisting ofF, S, and P.

[0048] The amount of the doping element is in a range of 0.0001 to 20 wt%, and preferably in the range of 0.01 to 10 wt % of the positive activematerial, the lithiated intercalation compound. When the amount thereofis below 0.0001 wt %, the effect of doping may not be sufficiently highto be effective. When the amount of the doping element is above 20 wt %,the electrode capacity may be reduced.

[0049] The positive active material is applied to a positive electrodeof a lithium secondary battery as follows: a positive active materialslurry is prepared by mixing the positive active material, bindermaterial, and conductive agent in an organic solvent. The positiveelectrode is generally fabricated by casting (coating) the slurry on acurrent collector and drying it, and then compressing the coated currentcollector.

[0050] A cross-sectional view of a prismatic lithium secondary batterycell according to an embodiment of the present invention is illustratedin FIG. 1. As shown in FIG. 1, the lithium secondary battery 1 isfabricated by the following process. An electrode assembly 8 is preparedby winding a positive electrode 2, a negative electrode 4, and aseparator 6 interposed between the positive and negative electrodes 2and 4, then placing the electrode assembly 8 into a battery case 10. Anelectrolyte is injected in the case 10, and the upper part of thebattery case 10 is sealed. In the battery, a conventional negativeelectrode 4 and electrolyte can be used. The negative electrode 4comprises a material that can reversibly deintercalate/intercalatelithium ions, such as a carbonaceous material. The electrolyte compriseslithium salts and organic solvents. It is understood that other types ofbatteries can be constructed using the coated active material of thepresent invention. Further, it is understood that, when the electrolyteis a solid electrolyte, the separator 6 and the electrolyte need not beincluded separately.

[0051] The present invention is further explained in more detail withreference to the following examples. These examples, however, should notin any sense be interpreted as limiting the scope of the presentinvention.

COMPARATIVE EXAMPLE 1

[0052] LiOH.H₂O and Co₃O₄ powders were weighed in a Li/Co equivalentratio of 1/1 and mixed in a mortar grinder. The resultant mixture wassubjected to a first heat-treatment for 5 hours at 450° C. while purgingwith a stream of dry air at 3 liter/min, and then it was uniformly mixedin a mortar grinder at room temperature. A second heat-treatment wasperformed for 10 hours at 800° C. while purging with a stream of dry airat 3 liter/minutes to produce a positive active material, LiCoO₂ powder.The LiCoO₂ powder was sieved using a −325 mesh.

[0053] The resultant LiCoO₂ powder for a positive active material, KF1300 for a binder material, and Super P for a conductive agent weremixed in a weight ratio of 96/2/2 in N-methyl pyrrolidone to prepare apositive active material slurry. The positive active material slurry wascast onto a 25 μm thick Al foil in a thickness of about 100 μm, followedby drying and compressing the coated Al foil. The resultantslurry-coated Al foil was cut into a disk having a diameter of 1.6 cm(area of 2 cm²) to prepare a positive electrode.

[0054] Using the positive electrode and a lithium counter electrode, a2016 coin-type half-cell was fabricated in an Ar-purged glove box. Forthe electrolyte, a 1 M LiPF₆ solution in ethylene carbonate and dimethylcarbonate (1:1 volume ratio) was used.

COMPARATIVE EXAMPLE 2

[0055] A coin-type half-cell was fabricated by the same procedure as inComparative Example 1, except that the equivalent ratio of Li/Co was1/0.98, and the resulting positive active material was LiCo_(0.98)O₂powder.

COMPARATIVE EXAMPLE 3

[0056] A coin-type half-cell was fabricated by the same procedure as inComparative Example 1, except that LiOH.H₂O, Co₃O₄, and Al₂O₃ powderswere used at a Li/Co/Al equivalent ratio of 1/1/0.1, and the resultingpositive active material was LiCoAl_(0.1)O₂ powder.

COMPARATIVE EXAMPLE 4

[0057] Ni(NO₃)₂ and Mn(NO₃)₂ were respectively dissolved in water, toprepare a Ni-containing solution and a Mn-containing solution. The twosolutions were mixed, and NH₄OH was added to control pH of the mixedsolution. Ni and Mn were co-precipitated to prepareNi_(0.8)Mn_(0.2)(OH)₂.

[0058] LiOH and Ni_(0.8)Mn_(0.2)(OH)₂powders were weighed in aLi/(Ni+Mn) equivalent ratio of 1/1, and mixed in a mortar grinder inethanol for 30 minutes The resultant mixture was subjected to a firstheat-treatment for 10 hours at 450° C., and was mixed uniformly in amortar grinder after being cooled to room temperature. A secondheat-treatment was performed for 10 hours at 750° C., to produce apositive active material, LiNi_(0.8)Mn_(0.2)O₂ powder. TheLiNi_(0.8)Mn_(0.2)O₂ powder was then sieved using a −325 mesh.

[0059] The resultant LiNi_(0.8)Mn_(0.2)O₂ powder for a positive activematerial, KF 1300 for a binder material, and Super P for a conductiveagent were mixed in a weight ratio of 94/3/3 in N-methyl pyrrolidone toprepare a positive active material slurry. Using the slurry, a coincell-type half-cell was fabricated by the same procedure as inComparative Example 1.

COMPARATIVE EXAMPLE 5

[0060] Ni(NO₃)₂, Mn(NO₃)₂, and Co(NO₃)₂ were respectively dissolved towater to prepare a Ni-containing solution, a Mn-containing solution, anda Co-containing solution. The three solutions were mixed, and NH₄OH wasadded to control pH of the mixed solution. Ni, Mn, and Co wereco-precipitated to prepare Ni_(0.8)Mn_(0.1)Co_(0.1)(OH)₂.

[0061] LiOH and Ni_(0.8)Mn_(0.1)Co_(0.1)(OH)₂ powders were weighed in aLi/(Ni+Mn+Co) equivalent ratio of 1.03/1 and mixed in ethanol for 30minutes by ball-milling. The resultant mixture was subjected to a firstheat-treatment for 5 hours at 500° C., and it was then mixed uniformlyin a mortar grinder after being cooled to room temperature. A secondheat-treatment was performed for 15 hours at 800° C. to produce apositive active material, Li_(1.03)Ni_(0.8)Mn_(0.1)Co_(0.1)O₂ powder.The Li_(1.03)Ni_(0.8)Mn_(0.1)Co_(0.1)O₂ powder was sieved using a −325mesh.

[0062] The resultant Li_(1.03)Ni_(0.8)Mn_(0.1)Co_(0.1)O₂ powder for apositive active material, KF 1300 for a binder material, and Super P fora conductive agent were mixed in a weight ratio of 94/3/3 in N-methylpyrrolidone to prepare a positive active material slurry. Using theslurry, a coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1.

COMPARATIVE EXAMPLE 6

[0063] Ni(NO₃)₂ and Co(NO₃)₂ were respectively dissolved in water toprepare a Ni-containing solution and a Co-containing solution. The twosolutions were mixed and NH₄OH was added to control pH of the mixedsolution. Ni and Co were coprecipitated to prepareNi_(0.9)Co_(0.1)(OH)₂.

[0064] LiOH and Ni_(0.9)Co_(0.1)(OH)₂powders were weighed in aLi/(Ni+Co) equivalent ratio of 1.05/1 and mixed in ethanol for 30minutes by ball-milling. The resultant mixture was subjected to a firstheat-treatment for 10 hours at 450° C. and it was then mixed uniformlyin a mortar grinder after being cooled to room temperature. A secondheat-treatment was performed for 10 hours at 770° C. to produce apositive active material, Li_(1.05)Ni_(0.9)Co_(0.1)O₂ powder. TheLi_(1.05)Ni_(0.9)Co_(0.1)O₂ powder was sieved using a −325 mesh.

[0065] The resultant Li_(1.05)Ni_(0.9)Co_(0.1)O₂ powder for a positiveactive material, KF 1300 for material, and Super P for a conductiveagent were mixed in a weight ratio of 94/3/3 in N-methyl pyrrolidone toprepare a positive active material slurry. Using the slurry, a coincell-type half-cell was fabricated by the same procedure as inComparative Example 1.

COMPARATIVE EXAMPLE 7

[0066] Mn(NO₃)₂ and Ni(NO₃)₂ were dissolved to water to prepare aMn—Ni-containing solution. NH₄OH was added to control pH of thesolution. Mn and Ni were coprecipitated to prepareMn_(0.75)Ni_(0.25)(OH)₂.

[0067] LiOH, Mn_(0.75)Ni_(0.25)(OH)₂, and Al₂O₃ powders were weighed ina Li/(Mn+Ni)/Al equivalent ratio of 1/2/0.03, and mixed in a mortargrinder in ethanol for 30 minutes The resultant mixture was subjected toa first heat-treatment for 10 hours at 450° C., and it was then mixeduniformly in a mortar grinder after being cooled to room temperature. Asecond heat-treatment was performed for 15 hours at 750° C. to produce apositive active material, LiMn_(1.5)Ni_(0.5)Al_(0.03)O₄ powder. TheLiMn_(1.5)Ni_(0.5)Al_(0.03)O₄ powder was sieved using meshes (Mesh No.325).

[0068] The resultant LiMn_(1.5)Ni_(0.5)Al_(0.03)O₄ powder for a positiveactive material, KF 1300 for a binder material, and Super P for aconductive agent were mixed in a weight ratio of 94/3/3 in N-methylpyrrolidone to prepare a positive active material slurry. Using theslurry, a coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1.

COMPARATIVE EXAMPLE 8

[0069] A coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1, except that LiOH, Mn_(0.75)Ni_(0.25)(OH)₂,and Al₂O₃ powders were mixed in a Li/(Mn+Ni)/Al equivalent ratio of1/2/0.7 to produce a positive active material,LiMn_(1.5)Ni_(0.5)Al_(0.7)O₄ powder.

[0070] The following Reference Examples are embodiments of Korean patentapplication No. 2001-31530 where the lithium source, metal source, anddoping-element-containing suspension were mixed simultaneously and themixture was heat-treated to prepare a positive active material withdoping elements.

Reference Example 1

[0071] A 5 wt % Al-isopropoxide suspension was prepared by adding 5 wt %of Al-isopropoxide powder to 95 wt % of ethanol.

[0072] LiOH.H₂O and Co₃O₄ powders and an Al-isopropoxide suspension wereweighed in a Li/Co/Al equivalent ratio of 1/1/0.05, and mixed in amortar grinder until all ethanol was evaporated. The resultant mixturewas subjected to a first heat-treatment for 5 hours at 450° C. whilepurging with a stream of dry air, and it was then mixed uniformly in amortar grinder after being cooled to room temperature. A secondheat-treatment was performed for 10 hours at 800° C. while purging witha stream of dry air to produce a positive active material,LiCoAl_(0.05)O₂ powder. The LiCoAl_(0.05)O₂ powder was sieved usingmeshes (Mesh No. 325).

[0073] The resultant LiCoAl_(0.05)O₂ powder for a positive activematerial, polyvinylidene fluoride for a binder material, and a carbonconductive agent were mixed in a weight ratio of 96/2/2 in N-methylpyrrolidone to prepare a positive active material slurry. Using theslurry, a coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1.

Reference Example 2

[0074] A 5 wt % Al-isopropoxide suspension was prepared by adding 5 wt %of Al-isopropoxide powder to 95 wt % of ethanol.

[0075] LiOH.H₂O and Co₃O₄ powders and the Al-isopropoxide suspensionwere weighed in a Li/Co/Al equivalent ratio of 1/1/0.1, and mixed in amortar grinder until all ethanol was evaporated. The resultant mixturewas subjected to a first heat-treatment for 5 hours at 500° C. whilepurging with a stream of dry air, and it was then mixed uniformly in amortar grinder after being cooled to room temperature. A secondheat-treatment was performed for 10 hours at 850° C. while purging witha stream of dry air to produce a positive active material,LiCoAl_(0.1)O₂ powder. The LiCoAl_(0.1)O₂ powder was sieved using meshes(Mesh No. 325).

[0076] The resultant LiCoAl_(0.1)O₂ powder for a positive activematerial, polyvinylidene fluoride for a binder material, and a carbonconductive agent were mixed in a weight ratio of 96/2/2 in N-methylpyrrolidone to prepare a positive active material slurry. Using theslurry, a coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1.

Reference Example 3

[0077] A 5 wt % Al-isopropoxide suspension was prepared by adding 5 wt %of Al-isopropoxide powder to 95 wt % of ethanol.

[0078] LiOH.H₂O and Co₃O₄ powders and the Al-isopropoxide suspensionwere weighed in a Li/Co/Al equivalent ratio of 1/0.95/0.05, and mixed ina mortar grinder until all ethanol was evaporated. The resultant mixturewas subjected to a first heat-treatment for 5 hours at 450° C. whilepurging with a stream of dry air, and it was then mixed uniformly in amortar grinder after being cooled to room temperature. A secondheat-treatment was performed for 10 hours at 800° C. while purging witha stream of dry air to produce a positive active material,LiCo_(0.95)Al_(0.05)O₂ powder. The LiCo_(0.95)Al_(0.05)O₂ powder wassieved using meshes (Mesh No. 325).

[0079] The resultant LiCo_(0.95)Al_(0.05)O₂ powder for a positive activematerial, polyvinylidene fluoride for a binder material, and a carbonconductive agent were mixed in a weight ratio of 96/2/2 in N-methylpyrrolidone to prepare a positive active material slurry. Using theslurry, a coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1.

Reference Example 4

[0080] A 5 wt % Al-isopropoxide suspension was prepared by adding 5 wt %of Al-isopropoxide powder to 95 wt % of ethanol.

[0081] LiOH.H₂O and Co₃O₄ powders and the Al-isopropoxide suspensionwere weighed in a Li/Co/Al equivalent ratio of 1/0.97/0.03, and mixed ina mortar grinder until all ethanol was evaporated. The resultant mixturewas subjected to a first heat-treatment for 5 hours at 500° C. whilepurging with a stream of dry air, and it was then mixed uniformly in amortar grinder after being cooled to room temperature. A secondheat-treatment was performed for 10 hours at 775° C. while purging witha stream of dry air to produce a positive active material,LiCo_(0.97)Al_(0.03)O₂powder. The LiCo_(0.97)Al_(0.03)O₂ powder wassieved using meshes (Mesh No. 325).

[0082] The resultant LiCo_(0.97)Al_(0.03)O₂powder for a positive activematerial, polyvinylidene fluoride for a binder material, and a carbonconductive agent were mixed in a weight ratio of 96/2/2 in N-methylpyrrolidone to prepare a positive active material slurry. Using theslurry, a coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1.

EXAMPLE 1

[0083] A 5 wt % Al-isopropoxide suspension was prepared by adding 5 wt %of Al-isopropoxide powder to 95 wt % of ethanol.

[0084] LiOH.H₂O and Co₃O₄ powders and the Al-isopropoxide suspensionwere weighed in a Li/Co/Al equivalent ratio of 1/1/0.1, and the Co₃O₄powder was added to the Al-isopropoxide suspension. The mixture wasmixed in a mortar grinder and dried until all ethanol was evaporated toobtain Co₃O₄ surface-treated with the Al-containing compound.

[0085] The surface-treated Co₃O₄ and the LiOH.H₂O were mixed byball-milling for 2 hours. The resultant mixture was subjected to a firstheat-treatment for 5 hours at 450° C. while purging with a stream of dryair, and it was then mixed uniformly in a mortar grinder after beingcooled to room temperature. A second heat-treatment was performed for 10hours at 800° C. while purging with a stream of dry air to produce apositive active material, LiCo_(0.95)Al_(0.05)O₂ powder. TheLiCo_(0.95)Al_(0.05)O₂ powder was sieved using meshes (Mesh No. 325).

[0086] The resultant LiCo_(0.95)Al_(0.05)O₂ powder for a positive activematerial, polyvinylidene fluoride for a binder material, and a carbonconductive agent were mixed in a weight ratio of 96/2/2 in N-methylpyrrolidone to prepare a positive active material slurry. Using theslurry, a coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1.

EXAMPLE 2

[0087] A coin cell-type half-cell was fabricated by the same procedureas in Example 1, except that the equivalent ratio of Li/Co/Al was1/1/0.05 to produce a positive active material, LiCoAl_(0.05)O₂ powder.

EXAMPLE 3

[0088] A coin cell-type half-cell was fabricated by the same procedureas in Example 1, except that the equivalent ratio of Li/Co/Al was1/0.95/0.05, and the second heat-treatment was performed for 10 hours at850° C., to produce a positive active material, LiCo_(0.95)Al_(0.05)O₂powder.

EXAMPLE 4

[0089] A coin cell-type half-cell was fabricated by the same procedureas in Example 1, except that the equivalent ratio of Li/Co/Al was1/0.93/0.07 to produce a positive active material,LiCo_(0.93)Al_(0.07)O₂ powder.

EXAMPLE 5

[0090] A 5 wt % Al-isopropoxide suspension was prepared by adding 5 wt %of Al-isopropoxide powder to 95 wt % of ethanol.

[0091] LiOH.H₂O and Co₃O₄ powders and the Al-isopropoxide suspensionwere weighed in a Li/Co/Al equivalent ratio of 1/1/0.05. The Co₃O₄powder was added to the Al-isopropoxide suspension. The mixture wasmixed in a mortar grinder until all ethanol was evaporated, and it wasthen subjected to a preheat treatment for 5 hours at 500° C. to obtain aCo—Al-containing solid solution compound powder.

[0092] The Co—Al-containing solid solution compound powder and theLiOH.H₂O were mixed by ball-milling for 2 hours. The resultant mixturewas subjected to a first heat-treatment for 5 hours at 450° C. whilepurging with a stream of dry air, and it was then mixed uniformly in amortar grinder after being cooled to room temperature. A secondheat-treatment was performed for 10 hours at 800° C. while purging witha stream of dry air to produce a positive active material,LiCoAl_(0.05)O₂ powder. The LiCoAl_(0.05)O₂ powder was sieved usingmeshes (Mesh No. 325).

[0093] The resultant LiCoAl_(0.05)O₂ powder for a positive activematerial, polyvinylidene fluoride for a binder material, and a carbonconductive agent were mixed in a weight ratio of 96/2/2 in N-methylpyrrolidone to prepare a positive active material slurry. Using theslurry, a coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1.

EXAMPLE 6

[0094] A coin cell-type half-cell was fabricated by the same procedureas in Example 5, except that the equivalent ratio of Li/Co/Al was1/1/0.01 and the second heat-treatment was performed for 10 hours at775° C. to produce a positive active material, LiCoAl_(0.01)O₂ powder.

EXAMPLE 7

[0095] A coin cell-type half-cell was fabricated by the same procedureas in Example 5, except that the equivalent ratio of Li/Co/Al was1/0.95/0.05 to produce a positive active material,LiCo_(0.95)Al_(0.05)O₂ powder.

EXAMPLE 8

[0096] A coin cell-type half-cell was fabricated by the same procedureas in Example 5, except that the equivalent ratio of Li/Co/Al was1/0.9/0.1 to produce a positive active material, LiCo_(0.9)Al_(0.1)O₂powder.

EXAMPLE 9

[0097] A 5 wt % B-containing suspension was prepared by adding 5 wt % ofB₂O₃ powder to 95 wt % of water.

[0098] LiOH.H₂O and Co₃O₄ powders and the B-containing suspension wereweighed in a Li/Co/B equivalent ratio of 1/1/0.1. The Co₃O₄ powder wasadded to the B-containing suspension. The mixture was mixed in a mortargrinder and dried until all water was evaporated, and it was then driedfurther in an oven for 5 hours to obtain Co₃O₄ surface-treated with theB-containing compound.

[0099] The surface-treated Co₃O₄ and the LiOH.H₂O were mixed byball-milling for 2 hours. The resultant mixture was subjected to a firstheat-treatment for 5 hours at 500° C. while purging with a stream of dryair, and it was then mixed uniformly in a mortar grinder after beingcooled to room temperature. A second heat-treatment was performed for 10hours at 800° C. while purging with a stream of dry air to produce apositive active material, LiCoB_(0.1)O₂ powder. The LiCoB_(0.1)O₂ powderwas sieved using meshes (Mesh No. 325).

[0100] The resultant LiCoB_(0.1)O₂powder for a positive active material,polyvinylidene fluoride for a binder material, and a carbon conductiveagent were mixed in a weight ratio of 96/2/2 in N-methyl pyrrolidone toprepare a positive active material slurry. Using the slurry, a coincell-type half-cell was fabricated by the same procedure as inComparative Example 1.

EXAMPLE 10

[0101] A coin cell-type half-cell was fabricated by the same procedureas in Example 9, except that the equivalent ratio of Li/Co/B was1/0.93/0.07, and the second heat-treatment was performed for 10 hours at825° C. to produce a positive active material, LiCo_(0.93)B_(0.07)O₂powder.

EXAMPLE 11

[0102] A 5 wt % B-containing suspension was prepared by adding 5 wt % ofB₂O₃ powder to 95 wt % of water.

[0103] LiOH.H₂O and Co₃O₄ powders and the B-containing suspension wereweighed in a Li/Co/B equivalent ratio of 1/1/0.05. The Co₃O₄ powder wasadded to the B-containing suspension. The mixture was mixed in a mortargrinder and dried until all water was evaporated, and it was then driedfurther in an oven for 5 hours. Subsequent to drying, preheat treatmentwas performed for 5 hours at 500° C. to obtain Co—B-containing-solidsolution compound powder.

[0104] The Co—B-containing solid solution compound powder and theLiOH.H₂O were mixed by ball-milling for 2 hours. The resultant mixturewas subjected to a first heat-treatment for 5 hours at 450° C. whilepurging with a stream of dry air, and it was then mixed uniformly in amortar grinder after being cooled to room temperature. A secondheat-treatment was performed for 10 hours at 800° C. while purging witha stream of dry air to produce a positive active material,LiCoB_(0.05)O₂ powder. The LiCoB_(0.05)O₂ powder was sieved using meshes(Mesh No. 325).

[0105] The resultant LiCoB_(0.05)O₂ powder for a positive activematerial, polyvinylidene fluoride for a binder material, and a carbonconductive agent were mixed in a weight ratio of 96/2/2 in N-methylpyrrolidone to prepare a positive active material slurry. Using theslurry, a coin cell-type half-cell was fabricated by the same procedureas in Comparative Example 1.

EXAMPLE 12

[0106] A coin cell-type half-cell was fabricated by the same procedureas in Example 11, except that the equivalent ratio of Li/Co/B was1/0.9/0.1 to produce a positive active material, LiCo_(0.9)B_(0.1)O₂powder.

EXAMPLE 13

[0107] Mn(NO₃)₂ and Ni(NO₃)₂ were dissolved in water to prepare aMn—Ni-containing solution, and NH₄OH was added to control pH of thesolution. Mn and Ni were coprecipitated to prepareMn_(0.75)Ni_(0.25)(OH)₂.

[0108] A 5 wt % Al-isopropoxide suspension was prepared by adding 5 wt %of Al-isopropoxide powder to 95 wt % of ethanol.

[0109] LiOH.H₂O, Mn_(0.75)Ni_(0.25)(OH)₂, and the Al-isopropoxidesuspension were weighed in a Li/(Mn+Ni)/Al equivalent ratio of 1/2/0.01.The Mn_(0.75)Ni_(0.25)(OH)₂ powder was added to the Al-isopropoxidesuspension. The mixture was mixed in a mortar grinder and dried untilall ethanol was evaporated to obtain Mn_(0.75)Ni_(0.25)(OH)₂surface-treated with the Al-containing compound.

[0110] The surface-treated Mn_(0.75)Ni_(0.25)(OH)₂ and the LiOH.H₂O weremixed by ball-milling for 2 hours. The resultant mixture was subjectedto a first heat-treatment for 5 hours at 500° C. while purging with astream of dry air, and it was then mixed uniformly in a mortar grinderafter being cooled to room temperature. A second heat-treatment wasperformed for 10 hours at 800° C. while purging with a stream of dry airto produce a positive active material, LiMn_(1.5)Ni_(0.5)Al_(0.01)O₄powder. The LiMn_(1.5)Ni_(0.5)Al_(0.01)O₄ powder was sieved using meshes(Mesh No. 325).

[0111] The resultant LiMn_(1.5)Ni_(0.5)Al_(0.01)O₄ powder for a positiveactive material, polyvinylidene fluoride for a binder material, and acarbon conductive agent were mixed in a weight ratio of 96/2/2 inN-methyl pyrrolidone to prepare a positive active material slurry. Usingthe slurry, a coin cell-type half-cell was fabricated by the sameprocedure as in Comparative Example 1.

EXAMPLE 14

[0112] Ni(NO₃)₂ and Co(NO₃)₂ were dissolved in water to respectivelyprepare a Ni-containing solution and a Co-containing solution, and thetwo solutions were mixed and NH₄OH was added to control pH of the mixedsolution. Ni and Co were coprecipitated to prepareNi_(0.9)Co_(0.1)(OH)₂.

[0113] A 5 wt % Al-isopropoxide suspension was prepared by adding 5 wt %of Al-isopropoxide powder to 95 wt % of ethanol.

[0114] LiOH, Ni_(0.9)Co_(0.1)(OH)₂, and the Al-isopropoxide suspensionwere weighed in a Li/(Ni+Co)/Al equivalent ratio of 1/1/0.01. TheNi_(0.9)Co_(0.1)(OH)₂ powder was added to the Al-isopropoxidesuspension. The mixture was mixed in a mortar grinder and dried untilall ethanol was evaporated, and it was then subjected to a preheattreatment for 5 hours at 500° C. to obtain Ni—Co—Al-containing solidsolution compound powder.

[0115] The Ni—Co—Al-containing solid solution compound powder and theLiOH were mixed by ball-milling for 2 hours. The resultant mixture wassubjected to a first heat-treatment for 5 hours at 500° C. while purgingwith a stream of dry air, and it was then mixed uniformly in a mortargrinder after being cooled to room temperature. A second heat-treatmentwas performed for 10 hours at 800° C. while purging with a stream of dryair to produce a positive active material, LiNi_(0.9)Co_(0.1)Al_(0.01)O₂powder. The LiNi_(0.9)Co_(0.1)Al_(0.01)O₂ powder was sieved using meshes(Mesh No. 325).

[0116] The resultant LiNi_(0.9)Co_(0.1)Al_(0.01)O₂ powder for a positiveactive material, polyvinylidene fluoride for a binder material, and acarbon conductive agent were mixed in a weight ratio of 96/2/2 inN-methyl pyrrolidone to prepare a positive active material slurry. Usingthe slurry, a coin cell-type half-cell was fabricated by the sameprocedure as in Comparative Example 1.

EXAMPLE 15

[0117] Ni(NO₃)₂, Mn(NO₃)₂, and Co(NO₃)₂ were respectively dissolved inwater to prepare a Ni-containing solution, a Mn-containing solution, anda Co-containing solution. The three solutions were mixed, and NH₄OH wasadded to control pH of the mixed solution. Ni, Mn, and Co werecoprecipitated to prepare Ni_(0.8)Mn_(0.1)Co_(0.1)(OH)₂.

[0118] A 5 wt % Al-isopropoxide suspension was prepared by adding 5 wt %of Al-isopropoxide powder to 95 wt % of ethanol.

[0119] LiOH, Ni_(0.8)Mn_(0.1)Co_(0.1)(OH)₂, and the Al-isopropoxidesuspension were weighed in a Li/(Ni+Mn+Co)/Al equivalent ratio of1.03/1/0.01. The Ni_(0.8)Mn_(0.1)Co_(0.1)(OH)₂ powder was added to theAl-isopropoxide suspension. The mixture was mixed in a mortar grinderand dried until all ethanol was evaporated, and it was then subjected toa preheat treatment for 5 hours at 500° C. to obtainNi—Mn—Co—Al-containing solid solution compound powder.

[0120] The Ni—Mn—Co—Al-containing solid solution compound powder and theLiOH were mixed by ball-milling for 2 hours. The resultant mixture wassubjected to a first heat-treatment for 5 hours at 500° C. while purgingwith a stream of dry air, and it was then mixed uniformly in a mortargrinder after being cooled to room temperature. A second heat-treatmentwas performed for 10 hours at 800° C. while purging with a stream of dryair to produce a positive active material,Li_(1.03)Ni_(0.8)Mn_(0.1)Co_(0.1)Al_(0.01)O₂ powder. TheLi_(1.03)Ni_(0.8)Mn_(0.1)Co_(0.1)Al_(0.01)O₂ powder was sieved usingmeshes (Mesh No. 325).

[0121] The resultant Li_(1.03)Ni_(0.8)Mn_(0.1)Co_(0.1)Al_(0.01)O₂ powderfor a positive active material, polyvinylidene fluoride for a bindermaterial, and a carbon conductive agent were mixed in a weight ratio of96/2/2 in N-methyl pyrrolidone to prepare a positive active materialslurry. Using the slurry, a coin cell-type half-cell was fabricated bythe same procedure as in Comparative Example 1.

[0122] In order to evaluate the charge-discharge characteristics of thecoin-type half-cells of Comparative Examples, Reference Examples, andExamples at various rates, the cells were charged-discharged. Results ofthe charge-discharge characteristics at various current densities (0.1Cand 1C) of Comparative Example 1, Reference Example 3, and Examples 2,3, 5, and 7 of the present invention are respectively shown in FIGS. 2and 3. As shown in FIG. 2, the discharge characteristics of the cells ofExamples 2, 3, 5, and 7 do not differ from those of Comparative Example1 or Reference Example 3 at a low rate of 0.1C. However, the cells ofExamples 2, 3, 5, and 7 are superior in discharge characteristics (i.e.,in discharge potential and discharge capacity) to those of ComparativeExample 1 and Reference Example 3, as shown in FIG. 3. When the currentdensity is increased from a low rate (0.1 C) to a high rate (1.0 C), thesuperiority is particularly pronounced.

[0123]FIG. 4A shows mid-point voltages of Comparative Example 1,Reference Example 3, and Examples 2, 3, 5, and 7, which are voltageswhere discharge capacity is 50% of maximum discharge capacity at a 1 Crate. FIG. 4B is an enlarged view of FIG. 4A. The results of FIGS. 4Aand 4B are represented in Table 1. TABLE 1 mid-point Difference based onvoltages (V) Comparative Example 1 (V) Comparative Example 1 3.834 0Reference Example 3 3.857 0.023 Example 2 3.858 0.024 Example 3 3.8570.023 Example 5 3.871 0.037 Example 7 3.874 0.040

[0124] As shown in Table 1, mid-point voltages of Examples were higherthan that of Comparative Example 1 by 0.023 to 0.040V. Enhancement ofthe mid-point voltage indicates that the power capabilities wereimproved.

[0125]FIG. 5 shows the results of charge-discharge cycling of the cellsof Comparative Example 1, Reference Example 2, and Examples 2, 5, and 7of the present invention at a 0.1C rate (1 cycle), a 0.2C rate (3cycles), a 0.5C rate (10 cycles), and a 1C rate (16 cycles). Asindicated in FIG. 5, the cells of Examples 2, 5, and 7 have superiorcycle-life characteristics to that of Comparative Example 1 by about 5%.

[0126] A portion of slurry according to Comparative Examples andExamples were placed in one location for 24 hours to evaluatecharacteristics of the slurries. They were evaluated visually as towhether gelation occurred, and the results are shown in Table 2. TABLE 1Characteristics of Slurries Comparative Example 4 Gelation ComparativeExample 5 Gelation Comparative Example 6 Gelation Comparative Example 7Gelation Comparative Example 8 Gelation Example 13 Good Example 14 GoodExample 15 Good

[0127] As shown in Table 2, the slurries of Comparative Examples 4 to 8gelled after 24 hours, while the slurries of Examples 13 to 15 had goodcharacteristics without occurrence of gelation.

[0128] The positive active material prepared in accordance with thepresent invention improves electrochemical characteristics includingcycle life, discharge potential, and power capability of a lithiumsecondary battery.

[0129] While the present invention has been described in detail withreference to the embodiments, those skilled in the art will appreciatethat various modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the present invention as setforth in the accompanying claims and equivalents thereof.

What is claimed is:
 1. A process of preparing a positive active material for a lithium secondary battery, comprising the steps of: adding a metal source to a doping element-containing coating liquid to surface-treat the metal source, wherein the metal source is selected from the group consisting of cobalt source, manganese source, nickel source, and a combination thereof; drying the surface-treated metal source to prepare a positive active material precursor; mixing the positive active material precursor with a lithium source; and heat-treating the mixture to produce the positive active material.
 2. The process of claim 1, wherein the doping element is soluble or suspendable in an organic solvent or water.
 3. The process of claim 2, wherein the doping element is at least one selected from the group consisting of Mg, Al, Co, Ni, K, Na, Ca, Si, Fe, Cu, Zn, Ti, Sn, V, Ge, Ga, B, P, Se, Bi, As, Zr, Mn, Cr, Sr, a rare earth element, and a combination thereof.
 4. The process of claim 3, wherein the doping element is at least one selected from the group consisting of Mg, Al, Co, Ni, Fe, Zr, Mn, Cr, Sr, V, and a combination thereof.
 5. The process of claim 1, wherein the metal source is a material including nickel or manganese-nickel.
 6. The process of claim 1, wherein an amount of the doping-element is in a range from 0.01 to 20 percent by weight of the doping-element-containing coating liquid.
 7. The process of claim 1, wherein the drying step is performed at a temperature in the range from room temperature to 300° C., for 1 to 24 hours.
 8. The process of claim 1, further comprising the step of preheating after the drying step to produce a solid solution compound including the doping element and the metal of the metal source.
 9. The process of claim 1, wherein the heat-treatment step comprises the steps of first heat-treating the mixture at 400° C. to 500° C. for 5 to 20 hours, and second heat-treating the mixture at 700° C. to 900° C. for 10 to 30 hours.
 10. The process of claim 1, wherein the positive active material is a compound having one of the following formulas (1) to (11): Li_(x)Mn_(1−y)M′_(z)A₂  (1)Li_(x)Mn_(1−y)M′_(z)O_(2−a)X_(a)  (2)Li_(x)Mn_(2−y)M′_(z)A₄  (3)Li_(x)Co_(1−y)M′_(z)A₂  (4)Li_(x)Co_(1−y)M′_(z)O_(2−a)X_(a)  (5)Li_(x)Ni_(1−y)M′_(z)A₂  (6)Li_(x)Ni_(1−y)M′_(z)O_(2−a)X_(a)  (7)Li_(x)Ni_(1−y)Co_(z)M′_(w)A_(b)  (8)Li_(x)Ni_(1−y)Co_(z)M′_(w)O_(2−b)X_(b)  (9)Li_(x)Ni_(1−y)Mn_(z)M′_(w)A_(b)  (10)Li_(x)Ni_(1−y−z)Mn_(z)M′_(w)O_(2−b)X_(b)  ( 1), wherein 0.9≦x≦1.1; 0≦y≦0.5; 0≦z≦0.5; 0≦w≦2; 0≦a≦0.5; 0≦b≦2; M′ is at least one doping element selected from the group consisting of Mg, Al, Co, Ni, K, Na, Ca, Si, Fe, Cu, Zn, Ti, Sn, V, Ge, Ga, B, P, Se, Bi, As, Zr, Mn, Cr, Sr, and a rare earth element; A is at least one element selected from the group consisting of O, F, S, and P; and X is at least one element selected from the group consisting of F, S, and P.
 11. A process of preparing a positive active material for a lithium secondary battery, comprising the steps of: adding a metal source to a doping element-containing coating liquid to surface-treat the metal source, wherein the metal source is selected from the group consisting of cobalt, manganese, nickel, and a combination thereof; preheat-treating the surface-treated metal source to prepare a positive active material precursor; mixing the positive active material precursor with a lithium source; and heat-treating the mixture to produce the positive active material.
 12. The process of claim 11, wherein the doping element is soluble or suspendable in an organic solvent or water.
 13. The process of claim 12, wherein the doping element is at least one selected from the group consisting of Mg, Al, Co, Ni, K, Na, Ca, Si, Fe, Cu, Zn, Ti, Sn, V, Ge, Ga, B, P, Se, Bi, As, Zr, Mn, Cr, Sr, Sc, Y, a rare earth element, and a combination thereof.
 14. The process of claim 13, wherein the doping element is at least one selected from the group consisting of Mg, Al, Co, Ni, Fe, Zr, Mn, Cr, Sr, V, and a combination thereof.
 15. The process of claim 11, wherein the metal source is a material including nickel or manganese-nickel.
 16. The process of claim 11, wherein an amount of the doping-element is in a range from 0.01 to 20 percent by weight of the doping-element-containing coating liquid.
 17. The process of claim 11, wherein the preheat-treatment step is performed at a temperature in the range from 300° C. to 1000° C., for 1 to 24 hours.
 18. The process of claim 11, wherein the heat-treatment step comprises the steps of first heat-treating the mixture at 400° C. to 500° C. for 5 to 20 hours, and second heat-treating the mixture at 700° C. to 900° C. for 10 to 30 hours.
 19. The process of claim 11, wherein the positive active material is a compound having one of the following formulas (1) to (11): Li_(x)Mn_(1−y)M′_(z)A₂  (1)Li_(x)Mn_(1−y)M′_(z)O_(2−a)X_(a)  (2)Li_(x)Mn_(2−y)M′_(z)A₄  (3)Li_(x)Co_(1−y)M′_(z)A₂  (4)Li_(x)Co_(1−y)M′_(z)O_(2−a)X_(a)  (5)Li_(x)Ni_(1−y)M′_(z)A₂  (6)Li_(x)Ni_(1−y)M′_(z)O_(2−a)X_(a)  (7)Li_(x)Ni_(1−y)Co_(z)M′_(w)A_(b)  (8)Li_(x)Ni_(1−y)Co_(z)M′_(w)O_(2−b)X_(b)  (9)Li_(x)Ni_(1−y)Mn_(z)M′_(w)A_(b)  (10)Li_(x)Ni_(1−y−z)Mn_(z)M′_(w)O_(2−b)X_(b)  ( 11), wherein 0.9≦x≦1.1; 0≦y≦0.5; 0≦z≦0.5; 0≦w≦2; 0≦a≦0.5; 0≦b≦2; M′ is at least one doping element selected from the group consisting of Mg, Al, Co, Ni, K, Na, Ca, Si, Fe, Cu, Zn, Ti, Sn, V, Ge, Ga, B, P, Se, Bi, As, Zr, Mn, Cr, Sr, and a rare earth element; A is at least one element selected from the group consisting of O, F, S, and P; and X is at least one element selected from the group consisting of F, S, and P.
 20. The process of claim 11, wherein an amount of the doping element is in a range from 0.0001 to 20 percent by weight of the positive active material.
 21. A process of preparing a positive active material for a lithium secondary battery, comprising the steps of: adding a nickel source and a manganese source to a doping element-containing coating liquid to surface-treat the nickel source and the manganese source; drying the surface-treated nickel source and manganese source to prepare a positive active material precursor; mixing the positive active material precursor with a lithium source; and heat-treating the mixture to produce the positive active material.
 22. A process of preparing a positive active material for a lithium secondary battery, comprising the steps of: adding a nickel source and a manganese source to a doping element-containing coating liquid to surface-treat the nickel source and the manganese source; preheat-treating the surface-treated nickel source and manganese source to prepare a positive active material precursor; mixing the positive active material precursor with a lithium source; and heat-treating the mixture to heat-treatment to produce the positive active material. 